Liquid-crystalline medium

ABSTRACT

Disclosed are liquid-crystalline mediums containing one or more compounds of formula I  
                 
and uses thereof for electro-optical purposes, e.g., to electro-optical liquid-crystal displays.

The present invention relates to a liquid-crystalline medium (LC medium)and to the use thereof for electro-optical purposes and to displayscontaining this medium.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (super-birefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Examples ofnon-linear elements which can be used to individually switch theindividual pixels are active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   1. MOS (metal oxide semiconductor) or other diodes on silicon wafers    as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out worldwide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are backlit.

The term MLC displays here encompasses any matrix display withintegrated non-linear elements, i.e., besides the active matrix, alsodisplays with passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket televisions) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. Besides problems regarding the angle dependence of thecontrast and the response times, difficulties also arise in MLC displaysdue to insufficiently high specific resistance of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, p. 145 ff, Paris]. With decreasingresistance, the contrast of an MLC display deteriorates, and the problemof after-image elimination may occur. Since the specific resistance ofthe liquid-crystal mixture generally drops over the life of an MLCdisplay owing to interaction with the interior surfaces of the display,a high (initial) resistance is very important in order to obtainacceptable lifetimes. In particular in the case of low-volt mixtures, itwas hitherto impossible to achieve very high specific resistance values.It is furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallisation and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not satisfytoday's requirements.

Besides liquid-crystal displays which use backlighting, i.e. areoperated transmissively and if desired transflectively, reflectiveliquid-crystal displays are also particularly interesting. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than backlitliquid-crystal displays having a corresponding size and resolution.Since the TN effect is characterised by very good contrast, reflectivedisplays of this type can even be read well in bright ambientconditions. This is already known of simple reflective TN displays, asused, for example, in watches and pocket calculators. However, theprinciple can also be applied to high-quality, higher-resolution activematrix-addressed displays, such as, for example, TFT displays. Here, asalready in the transmissive TFT-TN displays which are generallyconventional, the use of liquid crystals of low birefringence (Δn) isnecessary in order to achieve low optical retardation (d·Δn). This lowoptical retardation results in usually acceptably low viewing-angledependence of the contrast (cf. DE 30 22 818). In reflective displays,the use of liquid crystals of low birefringence is even more importantthan in transmissive displays since the effective layer thicknessthrough which the light passes is approximately twice as large inreflective displays as in transmissive displays having the same layerthickness.

Thus, there continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times, even at low temperatures, and a lowthreshold voltage which do not exhibit these disadvantages or only do soto a lesser extent.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

-   -   extended nematic phase range (in particular down to low        temperatures)    -   switchability at extremely low temperatures (outdoor use,        automobiles, avionics)    -   increased resistance to UV radiation (longer life)    -   low threshold voltage

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired whichfacilitate greater multiplexability and/or lower threshold voltagesand/or broader nematic phase ranges (in particular at low temperatures).To this end, a further widening of the available parameter latitude(clearing point, smectic-nematic transition or melting point, viscosity,dielectric parameters, elastic parameters) is urgently desired.

In particular in the case of LC displays in equipment for mobile videoapplications (for example mobile telephones, PDAs or notebooks withmultimedia functions, such as films or video games), a significantreduction in the response times is desired. At the same time, theoperating voltage should be as low as possible in order to reduce thetotal energy requirement of the equipment. However, it has been foundthat the LC media used for this purpose in the prior art often have aninadequate voltage holding ratio (HR) on exposure to light and heat, andinadequate low-temperature stability (LTS). This results in defects suchas streaks and so-called “mura” in the LC displays (see in this respect“Automatic blemish detection in liquid crystal flat panel displays” byWilliam K. Pratt et al., SPIE Proceedings 3306-01, pp. 2-13).

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not exhibit theabove-mentioned disadvantages or only do so to a lesser extent, andpreferably at the same time have high HR values, high low-temperaturestability, a low threshold voltage, a high clearing point and lowbirefringence. In addition, the LC media should have low rotationalviscosity in order to facilitate fast response times.

It has now been found that this object can be achieved if LC mediacomprising one or more compounds of the formula I are used. Thecompounds of the formula I result in mixtures having the desiredproperties indicated above.

The invention relates to a liquid-crystalline medium, characterised inthat it comprises one or more compounds of the formula I

in which

-   R⁰ denotes a halogenated or unsubstituted alkyl or alkoxy radical    having 1 to 15 C atoms, where, in addition, one or more CH₂ groups    in these radicals may each, independently of one another, be    replaced by —C≡C—, —CF₂O—, —CH═CH—,-    —O—, —CO—O— or —O—CO— in such a way that O atoms are not linked    directly to one another,-   X⁰ denotes F, Cl, CN, SF₅, SCN, NCS, a halogenated alkyl radical, a    halogenated alkenyl radical, a halogenated alkoxy radical or a    halogenated alkenyloxy radical having up to 6 C atoms, and-   Y^(1,2) each, independently of one another, denote H or F.

The compounds of the formula I have a high clearing point, high positivedielectric anisotropy, low birefringence and a broad nematic phaserange. Surprisingly, it has been found that LC media comprisingcompounds of the formula I have high LTS and a high HR at the same timeas a low threshold voltage, a high clearing point, low rotationalviscosity and consequently fast response times. They are thereforeparticularly suitable for LC displays in mobile and video applications.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, they can serve as basematerials of which liquid-crystalline media are predominantly composed;however, liquid-crystalline base materials from other classes ofcompound can also be added to the compounds of the formula I in order,for example, to modify the dielectric and/or optical anisotropy of adielectric of this type and/or to optimise its threshold voltage and/orits viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants known per se, which are notmentioned here in greater detail.

If R⁰ in the formulae above and below denotes an alkyl radical and/or analkoxy radical, this may be straight-chain or branched. It is preferablystraight-chain, has 2, 3, 4, 5, 6 or 7 C atoms and accordinglypreferably denotes ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy,propoxy, butoxy, pentoxy, hexyloxy or heptyloxy, furthermore methyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,methoxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy,tridecyloxy or tetradecyloxy.

Oxaalkyl preferably denotes straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R⁰ denotes an alkyl radical in which one CH₂ group has been replacedby —CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 C atoms. Accordingly, it denotes, inparticular, vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl,non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, dec-1-, -2-, -3-, -4-,-5-, -6-, -7-, -8- or -9-enyl.

If R⁰ denotes an alkyl or alkenyl radical which is at leastmonosubstituted by halogen, this radical is preferably straight-chain,and halogen is preferably F or Cl. In the case of polysubstitution,halogen is preferably F. The resultant radicals also includeperfluorinated radicals. In the case of monosubstitution, the fluorineor chlorine substituent may be in any desired position, but ispreferably in the ω-position.

In the formulae above and below, X⁰ is preferably F, Cl or mono- orpolyfluorinated alkyl or alkoxy having 1, 2 or 3 C atoms or mono- orpolyfluorinated alkenyl having 2 or 3 C atoms. X⁰ is particularlypreferably F, Cl, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃, OCFHCHF₂, OCFHCHF₂,OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂, OCF₂CF₂CHF₂, OCFHCF₂CF₃,OCFHCF₂CHF₂, OCF₂CF₂CF₃, OCF₂CF₂CClF₂, OCClFCF₂CF₃ or CH═CF₂, veryparticularly preferably F or OCF₃.

Particularly preferred compounds of the formula I are selected from thefollowing sub-formulae:

in which R⁰ and X⁰ have the meaning indicated in formula I.

R⁰ preferably denotes straight-chain alkyl having 1 to 8 C atoms,furthermore alkenyl having 2 to 7 C atoms. X⁰ preferably denotes F orOCF₃, particularly preferably F.

Further preferred embodiments are indicated below:

-   -   The medium additionally comprises one or more compounds of the        formulae II and/or III:    -   in which R⁰, X⁰, Y¹ and Y² have the meaning indicated in formula        I, Y³ and Y⁴ denote H or F, and    -   The compounds of the formula II are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F. Particular preference is given to compounds of the        formulae ha and lib;    -   The compounds of the formula III are preferably selected from        the following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F. Particular preference is given to compounds of the        formula IIIa;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R⁰, X⁰ and Y¹⁻⁴ have the meanings indicated in formula        II,        -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—,            —CH₂CF₂—, —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO— or —OCF₂—, in            formulae V and VI also a single bond, in formulae V and VIII            also —CF₂O—, and        -   r denotes 0 or 1;    -   The compounds of the formula IV are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F or OCF₃;    -   The compounds of the formula V are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F;    -   The compounds of the formula VI are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F;    -   The compounds of the formula VII are preferably selected from        the following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F;    -   The medium comprises one or more compounds selected from the        following formulae:    -   in which X⁰ has the meaning indicated above,        -   L denotes H or F,        -   “alkyl” denotes C₁₋₇-alkyl,        -   R′ denotes C₁₋₇-alkyl, C₁₋₆-alkoxy or C₂₋₇-alkenyl, and        -   “alkenyl” and “alkenyl*” each, independently of one another,            denote C₂₋₇-alkenyl.    -   The compounds of the formulae IX-XII are preferably selected        from the following formulae:    -   in which “alkyl” has the meaning indicated above;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R¹ and R² each, independently of one another, denote        n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having        up to 9 C atoms, and preferably each, independently of one        another, denote alkyl having 1 to 8 C atoms;    -   The medium additionally comprises one or more compounds of the        following formula:    -   in which R⁰, X⁰ and Y^(1,2) have the meanings indicated in        formula I, and    -    and each, independently of one another, denote    -    where A and B do not simultaneously denote cyclohexylene;    -   The compounds of the formula XV are preferably selected from the        following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F;    -   The medium comprises one or more compounds of the following        formula:    -   in which R¹ and R² have the meaning indicated above, and        preferably each, independently of one another, denote alkyl        having 1 to 8 C atoms, and L denotes H or F;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R^(1,2) and Y^(1,2) have the meanings indicated above;    -   The medium comprises one or more compounds of the formula XVIII,        in which R¹ and/or R² denote alkenyl having 2 to 7 C atoms,        preferably those selected from the following formulae:    -   in which “alkyl” has the meaning indicated above;    -   The medium additionally comprises one or more compounds selected        from the following formulae:    -   in which R⁰ and X⁰ each, independently of one another, have one        of the meanings indicated above, and Y¹⁻⁴ each, independently of        one another, denote H or F. X⁰ is preferably F, Cl, CF₃, OCF₃ or        OCHF₂. R⁰ preferably denotes alkyl, alkoxy, oxaalkyl,        fluoroalkyl or alkenyl, each having up to 8 C atoms. Particular        preference is given to compounds of the formula XXI.    -   The compounds of the formula XXI are preferably selected from        the following formulae:    -   in which R⁰ and X⁰ have the meanings indicated above.        Preferably, R⁰ denotes alkyl having 1 to 8 C atoms, and X⁰        denotes F;    -   R⁰ is straight-chain alkyl or alkenyl having 2 to 7 C atoms;    -   X⁰ is F;    -   The medium comprises one, two or more compounds of the formula        I, in particular of the formula Ia, Ib or Ic;    -   The medium comprises 2-40% by weight, preferably 3-30% by        weight, particularly preferably 3-20% by weight, of compounds of        the formula I;    -   The medium comprises compounds selected from the formulae I, II,        III, IV, VI, IX-XII, XVIII and XXI;    -   The proportion of compounds of the formulae II, III, IV, VI,        IX-XII, XVIII and XXI in the mixture as a whole is 40 to 95% by        weight;    -   The medium comprises 5-60% by weight, particularly preferably        10-50% by weight, of compounds of the formula II;    -   The medium comprises 2-40% by weight, particularly preferably        5-30% by weight, of compounds of the formula III;    -   The medium comprises 1-30% by weight, particularly preferably        2-20% by weight, of compounds of the formula IV;    -   The medium comprises 2-30% by weight, particularly preferably        3-20% by weight, of compounds of the formula VI;    -   The medium comprises 2-40% by weight, particularly preferably        3-30% by weight, of compounds of the formulae IX-XII and XVIII;    -   The medium comprises 1-20% by weight, particularly preferably        1-15% by weight, of compounds of the formula XXI.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to XXIII,results in a significant increase in the light stability and in lowbirefringence values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life. At the same time, the mixtures exhibit very low thresholdvoltages and very good values for the VHR on exposure to UV.

The term “alkyl” or “alkyl*” encompasses straight-chain and branchedalkyl groups having 1-7 carbon atoms, in particular the straight-chaingroups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groupshaving 1-6 carbon atoms are generally preferred.

The term “alkenyl” or “alkenyl*” encompasses straight-chain and branchedalkenyl groups having 2-7 carbon atoms, in particular the straight-chaingroups. Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofparticularly preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” preferably encompasses straight-chain groupshaving a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term “oxaalkyl” or “alkoxy” preferably encompasses straight-chainradicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and meach, independently of one another, denote 1 to 6. m may also denote 0.Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and lower values of k₃₃/k₁₁ compared withalkyl and alkoxy radicals. The mixtures according to the invention aredistinguished, in particular, by high K, values and thus havesignificantly faster response times than the mixtures from the priorart.

The optimum mixing ratio of the compounds of the above-mentionedformulae depends substantially on the desired properties, on the choiceof the components of the above-mentioned formulae and on the choice ofany further components that may be present.

Suitable mixing ratios within the range indicated above can easily bedetermined from case to case.

The total amount of compounds of the above-mentioned formulae in themixtures according to the invention is not crucial. The mixtures cantherefore comprise one or more further components for the purposes ofoptimisation of various properties. However, the observed effect on thedesired improvement in the properties of the mixture is generallygreater, the higher the total concentration of compounds of theabove-mentioned formulae.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VIII (preferably II,III, IV and VI, in particular IIa, IIIa and VIa), in which X⁰ denotes F,OCF₃, OCHF₂, OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergisticaction with the compounds of the formula I results in particularlyadvantageous properties. In particular, mixtures comprising compounds ofthe formulae I, IIa, IIIa and VIa are distinguished by their lowthreshold voltage.

The individual compounds of the above-mentioned formulae and thesub-formulae thereof which can be used in the media according to theinvention are either known or can be prepared analogously to the knowncompounds.

The invention also relates to electro-optical displays, such as, forexample, STN or MLC displays, having two plane-parallel outer plates,which, together with a frame, form a cell, integrated non-linearelements for switching individual pixels on the outer plates, and anematic liquid-crystal mixture having positive dielectric anisotropy andhigh specific resistance located in the cell, which contain media ofthis type, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant broadening of the available parameter latitude. Theachievable combinations of clearing point, viscosity at low temperature,thermal and UV stability and high optical anisotropy are far superior toprevious materials from the prior art.

The mixtures according to the invention are particularly suitable formobile applications and low-Δn TFT applications, such as, for example,mobile telephones and PDAs.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., and the clearing point ≧70° C.,preferably ≧75° C., particularly preferably ≧80° C., at the same timeallow dielectric anisotropy values Δ∈≧+8, preferably ≧+10, and a highvalue for the specific resistance to be achieved, enabling excellent MLCdisplays to be obtained. In particular, the mixtures are characterisedby low operating voltages.

The threshold voltage of the liquid-crystal mixtures according to theinvention is preferably ≦1.5 V, particularly preferably ≦1.3 V.

The birefringence Δn of the liquid-crystal mixtures according to theinvention is preferably ≦0.11, particularly preferably ≦0.10.

The rotational viscosity γ₁ of the liquid-crystal mixtures according tothe invention at 20° C. is preferably ≦180 mPa·s, particularlypreferably ≦140 mPa·s.

The nematic phase range of the liquid-crystal mixtures according to theinvention preferably has a width of at least 90°, in particular at least100°. This range preferably extends at least from −20° to +75° C.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 100° C.) to be achieved athigher threshold voltages or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having a higher Δ∈ and thus lowthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistance values to be achieved using the mixturesaccording to the invention at the first minimum than in the case ofmixtures comprising cyano compounds. Through a suitable choice of theindividual components and their proportions by weight, the personskilled in the art is able to set the birefringence necessary for apre-specified layer thickness of the MLC display using simple routinemethods.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a significantlysmaller decrease in the HR on UV exposure than analogous mixturescomprising cyanophenylcyclohexanes of the formula

or esters of the formula

instead of the compounds of the formula I.

The light stability and UV stability of the mixtures according to theinvention are considerably better, i.e. they exhibit a significantlysmaller decrease in the HR on exposure to light or UV. Even lowconcentrations of the compounds (<10% by weight) of the formula I in themixtures increase the HR by 6% or more compared with mixtures from theprior art.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the usual design for displays of this type. The termusual design is broadly drawn here and also encompasses all derivativesand modifications of the MLC display, in particular including matrixdisplay elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the inventionand the hitherto conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se, for example bymixing one or more compounds of the formula I with one or more compoundsof the formulae II-XXIII or with further liquid-crystalline compoundsand/or additives. In general, the desired amount of the components usedin lesser amount is dissolved in the components making up the principalconstituent, advantageously at elevated temperature. It is also possibleto mix solutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature, such as, forexample, UV stabilisers, such as Tinuvin® from Ciba, antioxidants,free-radical scavengers, nanoparticles, etc. For example, 0-15% ofpleochroic dyes or chiral dopants can be added. Suitable stabilisers anddopants are mentioned below in Tables C and D.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetrans-formation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m C atoms respectively; n andm are integers and preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11 or 12. The coding in Table B is self-evident. In Table A, only theacronym for the parent structure is indicated. In individual cases, theacronym for the parent structure is followed, separated by a dash, by acode for the substituents R^(1*), R^(2*), L^(1*) and L^(2*). Code forR¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* nm C_(n)H_(2n+1) C_(m)H_(2m+1) HH nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO•m OC_(n)H_(2n+1) C_(m)H_(2m+1)H H n C_(n)H_(2n+1) CN H H nN•F C_(n)H_(2n+1) CN F H nN•F•FC_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nOFOC_(n)H_(2n+1) F H H nF•F C_(n)H_(2n+1) F F H nF•F•F C_(n)H_(2n+1) F F FnOCF₃ C_(n)H_(2n+1) OCF₃ H H nOCF₃•F C_(n)H_(2n+1) OCF₃ F H n-VmC_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H nV—Vm C_(n)H_(2n+1)—CH═CH——CH═CH—C_(m)H_(2m+1) H H

Preferred mixture components are found in Tables A and B. TABLE A

TABLE B

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three, four or more compounds from Table B. TABLE C Table C indicatespossible dopants which are generally added to the mix- tures accordingto the invention. The mixtures preferably comprise 0-10% by weight, inparticular 0.01-5% by weight and particularly preferably 0.01-3% byweight of dopants.

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention in amounts of 0-10% by weight are mentionedbelow.

The following examples are intended to explain the invention withoutlimiting it.

Above and below, percentage data denote percent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematicphase, S=smectic phase and I=isotropic phase. The data between thesesymbols represent the transition temperatures.

Furthermore,

-   -   An denotes the optical anisotropy at 589 nm and 20° C.,    -   ΔA denotes the rotational viscosity (mPa·s) at 20° C.,    -   V₁₀ denotes the voltage (V) for 10% of the change in        transmission (viewing angle perpendicular to the plate surface),        (threshold voltage),    -   V₉₀ denotes the voltage (V) for 90% of the change in        transmission (viewing angle perpendicular to the plate surface),    -   Δ∈ denotes the dielectric anisotropy at 20° C. and 1 kHz        (Δ∈=∈_(∥)−∈_(⊥), where ∈_(∥) denotes the dielectric constant        parallel to the longitudinal axes of the molecules and ∈_(⊥)        denotes the dielectric constant perpendicular thereto),    -   HR denotes the voltage holding ratio [%], and    -   LTS denotes the low-temperature stability (phase), determined in        test cells.

The electro-optical data are measured in a TN cell at the 1st minimum(i.e. at a d·Δn value of 0.5 μm) at 20° C., unless expressly indicatedotherwise. The optical data are measured at 20° C., unless expresslyindicated otherwise. All physical properties are determined inaccordance with “Merck Liquid Crystals, Physical Properties of LiquidCrystals”, status November 1997, Merck KGaA, Germany, and apply to atemperature of 20° C., unless explicitly indicated otherwise.

COMPARATIVE EXAMPLE 1

CC-4-V 18.00% Clearing point [° C.]: 80.0 PP-1-2V1  4.00% Δn [589 nm,20° C.]: 0.1000 CCQU-3-F 13.00% Δε [20° C., 1 kHz]: +10.5 CCQU-5-F12.00% γ₁ [mPa · s, 20° C.]: 107 CCP—20CF3  6.00% V₁₀ [V]: 1.28CCP—30CF3  2.00% LTS bulk −30 [h]: 24 PUQU-3-F 14.00% BCH—3F•F•F 16.00%CCP—V-1 8.00% CCGU-3-F  7.00%

COMPARATIVE EXAMPLE 2

CC-4-V 14.00%  Clearing point [° C.]: 80.5 CC-3-V1 2.00% Δn [589 nm, 20°C.]: 0.1000 CCQU-2-F 2.00% Δε [20° C., 1 kHz]: +11.4 CCQU-3-F 10.00%  γ₁[mPa · s, 20° C.]: 100 CCQU-5-F 10.00%  V₁₀ [V]: 1.24 CCP—20CF3 7.00%LTS bulk −30 [h]: 48 CCP—30CF3 6.00% PUQU-2-F 9.00% PUQU-3-F 12.00% PGU-3-F 7.00% CCG-V—F 4.00% CCP—V-1 15.00%  CCGU-3-F 2.00%

EXAMPLE 1

CC-4-V 13.00%  Clearing point [° C.]: 80.0 CC-3-V1 4.00% Δn [589 nm, 20°C.]: 0.0987 PP-1-2V1 1.00% Δε [20° C., 1 kHz]: +10.7 CCQU-3-F 10.00%  γ₁[mPa · s, 20° C.]: 99 CCQU-5-F 10.00%  V₁₀ [V]: 1.28 CCP—20CF3 4.00% LTSbulk −30 [h]: 1000 PUQU-2-F 7.00% HR [5 min, 100° C.]: 97.5% PUQU-3-F13.00%  PGU-3-F 6.00% CCG-V—F 7.00% CCP—V-1 13.00%  CCGU-3-F 2.00%CAP-3-F 10.00% 

The mixture has a significantly higher LTS compared with the mixturesfrom Comparative Examples 1 and 2, with comparable values for theclearing point, the birefringence, the dielectric anisotropy, therotational viscosity and the threshold voltage. In addition, it exhibitsa high HR value.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding German application No. 102006046905.4,filed Oct. 4, 2006 are incorporated by reference herein.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium, comprising one or more compounds offormula I

in which R⁰ denotes a halogenated or unsubstituted alkyl or alkoxyradical having 1 to 15 C atoms, in which optionally one or more CH₂groups are, independently of one another, replaced by —C≡C—, —CF₂O—,—CH═CH—,

 —O—, —CO—O— or —O—CO— in such a way that O atoms are not linkeddirectly to one another, X⁰ denotes F, Cl, CN, SF₅, SCN, NCS, ahalogenated alkyl radical, a halogenated alkenyl radical, a halogenatedalkoxy radical or a halogenated alkenyloxy radical having up to 6 Catoms, and Y¹ and Y² each, independently of one another, denote H or F.2. A liquid-crystalline medium according to claim 1, comprising one ormore compounds of formula Ia, Ib or Ic

in which R⁰ and X⁰ have the meanings indicated for formula I.
 3. Aliquid-crystalline medium according to claim 1, further comprising oneor more compounds of formula II or III

in which A, R⁰, X⁰, Y¹ and Y² have the meanings indicated for formula I,and Y³ and Y⁴ each, independently of one another, denote H or F.
 4. Aliquid-crystalline medium according to claim 3, comprising one or morecompounds of formula IIa, IIb, IIc, IId, IIIa, IIIb, IIIc or IIId

in which R⁰ and X⁰ have the meanings indicated for formula I.
 5. Aliquid-crystalline medium according to claim 1, further comprising oneor more compounds of formula IV, V, VI, VII or VIII

in which R⁰, X⁰ and Y¹ and Y² have the meanings indicated for formula I,Y³ and Y⁴ each, independently of one another, denote H or F, Z⁰ denotes—C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—,—OCH₂—, —COO— or —OCF₂—, in formulae V and VI also a single bond, informulae V and VIII also —CF₂O—, and r denotes 0 or
 1. 6. Aliquid-crystalline medium according to claim 1, further comprising oneor more compounds of formula IVa or IVb

in which R⁰ and X⁰ have the meanings indicated for formula I.
 7. Aliquid-crystalline medium according to claim 1, further comprising oneor more compounds of formula VIa, VIb, VIc or VId

in which R⁰ and X⁰ have the meanings indicated for formula I.
 8. Aliquid-crystalline medium according to claim 1, further comprising oneor more compounds of formula IX, X, XI, XII or XVIII

in which X⁰ has the meaning indicated for formula I, L is H or F,“alkyl” denotes C₁₋₇-alkyl, R′ denotes C₁₋₇-alkyl, C₁₋₆-alkoxy orC₂₋₇-alkenyl, “alkenyl” and “alkenyl*” each, independently of oneanother, denote C₂₋₇-alkenyl, and R¹ and R² each, independently of oneanother, denote n-alkyl, alkoxy, oxaalkyl, fluoroalkyl or alkenyl, eachhaving up to 9 C atoms, where at least one of the radicals R¹ and R²denotes alkenyl having 2 to 9 C atoms.
 9. A liquid-crystalline mediumaccording to claim 1, further comprising one or more compounds offormula XXIa or XXIb

in which R⁰ and X⁰ have the meaning indicated for formula I.
 10. Aliquid-crystalline medium according to claim 3, comprising 2-40% byweight of compounds of formula I, 20-90% by weight of compounds offormula II, 2-30% by weight of compounds of formula III, optionally2-30% by weight of compounds of formula IV

in which R⁰, X⁰ and Y¹ and Y² have the meanings indicated for formula I,optionally 2-30% by weight of compounds of formulae IX, X, XI and/or

in which X⁰ has the meaning indicated for formula I, L is H or F,“alkyl” denotes C₁₋₇-alkyl, R′ denotes C₁₋₇-alkyl, C₁₋₆-alkoxy orC₂₋₇-alkenyl, and “alkenyl” and “alkenyl*” each, independently of oneanother, denote C₂₋₇-alkenyl, and optionally 4-30% by weight ofcompounds of formula XIV

in which R¹ and R² each, independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 C atoms.11. An electro-optical apparatus comprising a liquid-crystalline mediumaccording to claim
 1. 12. An electro-optical liquid-crystal displaycontaining a liquid-crystalline medium according to claim
 1. 13. Aprocess for preparing a liquid-crystalline medium according to claim 1,comprising mixing together one or more compounds of formula I with oneor more further liquid-crystalline compounds and/or additives.
 14. Aliquid-crystalline medium according to claim 10, comprising 2-40% byweight of compounds of formula I, 20-90% by weight of compounds offormula II, 2-30% by weight of compounds of formula III, 2-30% by weightof compounds of formula IV

in which R⁰, X⁰ and Y¹ and Y² have the meanings indicated for formula I,2-30% by weight of compounds of formulae IX, X, XI and/or XII

in which X⁰ has the meaning indicated for formula I, L is H or F,“alkyl” denotes C₁₋₇-alkyl, R′ denotes C₁₋₇-alkyl, C₁₋₆-alkoxy orC₂₋₇-alkenyl, and “alkenyl” and “alkenyl*” each, independently of oneanother, denote C₂₋₇-alkenyl, and 4-30% by weight of compounds offormula XIV

in which R¹ and R² each, independently of one another, denote n-alkyl,alkoxy, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9 C atoms.15. A liquid-crystalline medium according to claim 1, which has anematic phase down to −20° C., a clearing point ≧70° C., and adielectric anisotropy value of Δ∈≧+8.
 16. A liquid-crystalline mediumaccording to claim 1, which has a threshold voltage of ≦1.5 V.
 17. Aliquid-crystalline medium according to claim 1, which has a thresholdvoltage of ≦1.3 V.
 18. A liquid-crystalline medium according to claim 1,which has a nematic phase range with a width of at least 90°, whichrange extends at least from −20° to +75° C.
 19. A liquid-crystallinemedium according to claim 1, which has a birefringence Δn of ≦0.11, or arotational viscosity γ₁ at 20° C. of ≦180 mPa·s.